1. Field of the Invention
The present invention relates to a correcting device and a correcting method for perspective transformation of document images, and more particularly, to a hybrid method combining the detection of vanishing points (including horizontal vanishing points and vertical vanishing points) by voting of various line segments and the detection of vanishing points by using image projection analysis, a method for searching vertical vanishing points by clustering based on vertical strokes of characters, and a method for correcting perspective transformation based on text knowledge.
2. Description of the Related Art
Perspective transformation correction of document images based on a digital camera is an important step in document analysis and recognition. The meaning of a perspective transformation correction operation is to convert images having perspective transformation into images without perspective transformation.
The principle of various perspective transformation corrections of document images may be referred to the article in Pattern Recognition 36(11), 2003, “Rectifying perspective views of text in 3D scenes using vanishing points” by P. Clark, M. Mirmehdi, the article in the international conference of IEEE Conference on Computer Vision and Patten Recognition 2001, “Extract of illusory linear clues in perspectively skewed documents” by M. Pilu, and the article in Image and Vision Computing 23(5), 2005, “Perspective rectification of document images using fuzzy set and morphological operations” by S. J. Lu, B. M. Chen, and C. C. Ko. Moreover, there have also been patents published specifically for correcting methods of document image perspective transformation, like U.S. Pat. No. 6,873,732, C. R. Dance, “Method and apparatus for resolving perspective distortion in a document image and for calculating line sums in images”, and 7,046,404, S. P. Cheatle, D. A. Grosvenor, “Document imaging system”, as well as Japanese Patent P2002-89496, , “Image processing apparatus and image processing method”. Perspective transformation correction of document images is usually performed on the basis of detection of the vanishing points, which includes detection of horizontal vanishing points and detection of vertical vanishing points Vanishing points detecting methods applied to image documents can be divided into two general classes. One directly uses image pixels projection analysis from a perspective view, and is referred to as the direct methods. This kind of methods involves high computational complexity and the speed of this kind of methods is very low. The other uses the analysis and voting of the line segments and their intersections indirectly, and is referred to as the indirect methods. This kind of methods has a high speed but is unstable for the processing performance.
Generally speaking, image documents captured with digital cameras, pickup lenses and they always have perspective transformation that would influence subsequent document analysis and recognition. Various algorithms for correcting perspective transformation of image documents have been proposed to correct these perspective transformations. These algorithms have effects to some extents. However, due to the different models of the digital cameras and pickup lenses as well as the different angles during the shooting, in particular with regard to document images captured with a pickup lens of a mobile phone, the circumstances of perspective transformations are diversified and complicated, to which no better solution has been obtained so far through the currently available correcting algorithms.